Abstract:

The invention relates to a controllable light modulator (M) whose
transmission can be controlled by the intensity of electric fields,
wherein it is exposed to an intensity-controlled microwave field.
Furthermore a device for laser projection is illustrated which is
controlled by a light modulator of this kind.

Claims:

1. Light modulator (M) with a light passage whose light transmission can
be controlled by the intensity of electromagnetic fields, characterized
in that it is exposed to an intensity-controlled microwave field.

2. Light modulator according to claim 1, characterized in that the light
passage is made of glass.

3. Light modulator according to claim 2, characterized in that the glass
is designed in such a way that the light transmission characteristic is
essentially determined by a microwave field in a specific frequency
range.

4. Light modulator according to claim 3, characterized in that the
microwave field has a defined frequency between 5 and 180 GHz.

5. Light modulator according to claim 4, characterized in that the
microwave frequency serves as a carrier frequency the amplitude of which
is modulated up to a frequency, that is approximately one order below the
microwave frequency.

6. Light modulator according to claim 2, characterized in that microwave
antennas (4a, 4b) are arranged in pairs around the light passage (3) on
one side or on several sides of the glass, with alternating polarity.

7. Light modulator according to claim 6, characterized in that the light
passage (3) has a diameter which is slightly larger than that of a laser
beam to be modulated.

8. Light modulator according to claim 6 characterized in that said that
the planar antenna electrodes (4a, 4b) function as capacitors and form
resonant circuits with inductors (5) arranged at the edges of the glass.

9. Light modulator according to claim 6, characterized in that the phase
of the waves in the circuits rotates around the light passage (3).

10. Light modulator according to claim 8, characterized in that the glass
is shaped as a cylinder and that the antenna electrodes (4a, 4b) form a
ring around the cylinder.

11. Device with a light modulator according to claim 1, characterized in
that it is exposed to a laser beam (L), the modulated light of which is
used in a messaging device with a light detector, or in a light recording
device, a processing device or a projection device.

12. Device according to claim 11, characterized in that the modulated
laser beam (L) is brought to an X- and Y-deflector (10, 11) which directs
it to a projection area.

13. Device according to claim 12, characterized in that the deflector
consists of two metallized prismatic cylinders (10, 11).

14. Device according claim 11, characterized in that several light
modulators (M), are each exposed to a laser (R, G, B) of different a
colour and that the resulting modulated laser beams (L) are superimposed
and directed to the deflector (10, 11).

15. Device according to claim 14, characterized in that the lasers (R, G,
B) emit light with a minimum of three primary colours.

16. Device according to claim 15, characterized in that each of the three
lasers (R, G, B) is modulated according to one of the RGB signals of a
video signal.

17. Device according to claim 14, characterized in that an additional
light modulator (M) is exposed to a white light beam (H), which is
modulated in accordance with a luminance signal.

18. Device according to claim 17, characterized in that the brightness of
the projection is controlled by the intensity of the white light beam
(H).

19. Device according to claim 17, characterized in that the white light
beam (H) is created from a blue laser beam by transformation in a filter
(F).

Description:

[0001]The invention relates to a controllable a light modulator element
with a light passage whose light transmission can be controlled by the
intensity of electromagnetic fields; and to its use, especially in a
colour imaging device.

[0002]It is well known to direct light through a polarizable medium, the
polarization properties of which can be changed by an electric or
magnetic field--a so-called Kerr-cell--, and through an unchanging
polarization filter. In dependence on the control of the cell, more or
less light passes these polarizers. Such Kerr cells require high electric
control voltages or strong magnetic fields, the generation of which is
relatively expensive, and they switch back to their original state with a
considerable delay, when the electric or magnetic excitation is
withdrawn. In addition, a substantial energy turnover is implemented in
the medium, if the excitation is changed at a high frequency.

[0003]Furthermore, colour imaging devices are known which employ three
intensity-controlled light sources of different colours, the light of
which is superimposed and displayed on a screen by means of moving X-
Y-deflectors.

[0004]Furthermore, solids are known, e.g. those implemented in welder's
protective goggles, which are dimmed by incoming light depending on its
intensity, so that their transmission strongly decreases with increasing
light intensity. This effect has a very short relaxation time. Flashing
lights do not pass through such a pane, but afterwards it is transparent
again.

[0005]It is the aim of the invention to create a controllable high-speed
light modulator.

[0006]The solution resides in that the light passage of the light
modulator element is exposed to an intensity-controlled microwave field.

[0007]Advantageous embodiments and implementations are indicated in the
subclaims.

[0008]The new light modulator element is particularly suitable for
controlling the intensity of a laser beam. The modulator element can
miniaturized due to the small diameter of a laser beam. The light passage
of the light modulator element is made of glass, the transmission
capacity of which can be controlled by the intensity of an alternating
electromagnetic field. The frequency and strength of the alternating
field depends on manufacturing parameters of the glass. So far, these
glasses are employed in spectacles such as sunglasses or welder's
goggles, in which the transparency (light transmission) depends on the
brightness of the light that is applied. Now this glass can also be
manufactured in such a way that instead of being controlled by light, the
transmission is controlled by electromagnetic fields with a defined
frequency, which is lower than the frequency of light radiation.
Typically, this is a frequency between 5 and 100 GHz. The transmission of
the glass directly depends on the applied field strength. This way, even
microwave fields can be used to control the transmission of the light
modulator.

[0009]In a preferential embodiment, microwave antennas are arranged in
pairs on one or both sides of the modulator element. A circular
arrangement of antennas with alternating polarity has proven particularly
successful. A quadrupolar field, an octopolar field, or the like, is thus
generated around a central light passage, which only needs to be slightly
larger than the modulating light beam.

[0010]In another execution the glass has a cylindrical shape and the
antenna electrodes are shaped as rings around the glass cylinder. The
electrodes employed for a microwave frequency of e.g. 50 GHz are only
millimetres in size.

[0011]Preferably, the electrode pairs are parts of capacitors, which
generate resonant circuits with inductive resistors arranged around the
modulator element. High field strengths result, due to the limited
thickness of the glass and the small gap between the electrodes. The
field strengths in the central light passage are further increased if the
phases of the resonant circuits are triggered in a staggered way, so that
the respective maximum phase rotates around the central light passage.

[0012]The light modulator can be operated with frequencies up to 180 GHz
if it is appropriately constructed. This frequency can serve as a carrier
frequency, which is modulated by a control frequency.

[0013]The intensity of a 50-GHz generator, for example, can be controlled
by a frequency of 5 GHz, and that frequency is also used for the
transmission of the light modulator element for a light beam or a laser
beam. A beam of a continuously operated laser that is controlled in this
way can be brought to varied uses, e.g. for an analogue or digital
message transmission, to record information, for material processing or,
as described in more detail, for image display. This method for
modulating a continuously operated laser avoids all known disadvantages
of pulsed lasers.

[0014]In a monochrome imaging device, a laser beam is directed through the
light modulator element, either directly or after its colour has been
modified by a filter, e.g. changed to white light. Then it is directed to
one and then another rotating prism mirror for X- and Y-deflection, and
projected onto a screen, where it creates an image in accordance with the
modulation of the light. To produce a video image, the control microwave
is operated with a monochrome video signal modulation and the rotating
metallized prisms are synchronized with the line- and image-change
signal.

[0015]Accordingly, a colour television image is generated by directing to
the prisms three superimposed modulated laser beams of different colours,
which are modulated according to the colour signals, i.e. the higher the
colour signal the lower the respective microwave energy.

[0016]In an advantageous embodiment, a white light beam is modulated
according to a luminance signal and added to the three colour laser beams
before they pass the prism. The white light beam is generated, in a known
simple way, from a blue laser beam by modification in a yellow filter.
The brightness of a projected image can be controlled by this additional
luminance signal, without having to change the output of the colour
lasers. Thus, colour shifts are avoided, that could otherwise occur--due
to the nonlinearity of the lasers--when the brightness of the image
changes.

[0017]A complete colour image projector of this type is accommodated in a
3 cm thick casing of DIN A5 dimensions and provides about 15 k Lumen. Due
to the high modulation frequency of 5 GHz that can be attained, images of
10 mega pixel at a picture repetition rate of 250 Hz can be generated
with unprecedented quality and brilliance.

[0018]In the figures, an execution of the invention is presented by way of
example.

[0019]FIG. 1 shows a schematic view of a light modulator

[0020]FIG. 2 shows a first electrode arrangement

[0021]FIG. 3 shows a second electrode arrangement

[0022]FIG. 4 shows a cylindrical arrangement

[0023]FIG. 5 shows a light modulator with generator

[0024]FIG. 6 shows a schematic view of a laser projector

[0025]In FIG. 1 a light modulator M is depicted schematically, in which a
central pane 2 is held. In the area of light passage 3 a laser beam L
passes the pane.

[0026]The circular antenna electrodes 4 are arranged on the pane 2, with
respectively two electrodes 4a, 4b forming a pair of electrodes. The
electric field strength is applied to the glass 2 by these electrodes 4a,
4b, which are part of a microwave resonant circuit, and the transmission
of the glass 2 is controlled.

[0027]Furthermore, the electrodes 4 serve to eliminate loss heat from the
glass.

[0028]FIG. 2 shows a first arrangement of electrodes 4a, 4b on the pane 2.
In this arrangement, respectively one pair of electrodes 4a, 4b is placed
on each side of the glass 2, forming a microwave resonant circuit with
the inductor 5. Because there is also a resonant circuit on the other
side, a quadrupolar field is generated. It is also possible that the
electrodes 4a, 4b are disposed only on one side of the glass 2, so that
dipole fields result.

[0029]FIG. 3 shows a second arrangement of electrodes 4a, 4b on the pane
2. In this arrangement, electrodes 4a, 4b on opposite sides of the glass
2 form pairs and form a microwave resonant circuit with the inductor 5.
Because of adjacent resonant circuits, a multipolar field is generated.

[0030]FIG. 4 depicts a light modulator element in cylindrical shape. The
two antenna electrodes 4a and 4b are laid in a ring around the glass
cylinder. They form the plates of a capacitor, which forms a resonant
circuit with the inductor 5, and between the electrodes of which an
alternating field results accordingly. This alternating field controls
the laser beam L, which is directed the axially through the glass
cylinder 2.

[0031]FIG. 5 is once again the schematic view of the modulator M according
to FIG. 1. The electrodes 4 on the pane 2 are activated by a
corresponding number of generators 6, of which only one is represented.
Each generator 6 feeds a circuit consisting of the inductors 5 and the
electrodes 4a, 4b. The intensity of the resulting microwave field is
controlled in accordance with the wanted signal N. The phase of the
generators 6 is controlled in such a way, that a rotating field is formed
on the pane 2, represented here by an arrow. This rotating field produces
a continuous control of the transmission in the light passage 3 for the
laser beam L.

[0032]In FIG. 6, a projector 1 with colour lasers R, G, B, W and
modulators M is shown schematically. The modulators M for the colour
laser R, G, B, are controlled in a known manner according to the colour
signals of an image (not shown here) and are combined to a colour beam by
mirrors 7 and prisms in the light superimposition 8. The beam of the
laser W, which is blue at first, is modulated in the corresponding
modulator M, according to a luminance signal. This luminance signal H is
changed to a white beam in a filter F and added to the colour beam 12 by
the prism 9. The brightness of the resulting image can be set by
appropriate modulation of the light signal H, without readjusting the
colour lasers R, G, B.

[0033]The colour beam 12 is deflected horizontally by the rotating
metallized prism cylinder 11, and deflected vertically by the rotating
metallized prism cylinder 10, in a known manner. The surfaces of the
prisms are inclined in such a way that the projection beam P takes a
straight course to the projection screen.